The present invention relates to a semiconductor memory implementable as a split-gate EEPROM, in which the control and floating gates of each storage element are formed side by side on a substrate, and also relates to a method for fabricating a memory of that type.
An electrically erasable and programmable read-only memory (EEPROM) with floating gates is known as a typical electrically erasable and programmable nonvolatile semiconductor memory.
Recently, a semiconductor memory should have its size further reduced to realize an even more densely integrated LSI and also has to have its performance further enhanced. A split-gate EEPROM was proposed as a structure that would contribute to such downsizing and performance enhancement greater than any other candidate. This is because a split-gate EEPROM includes a floating gate electrode that is horizontally adjacent to a control gate electrode with a capacitive insulating film interposed therebetween and because the split-gate EEPROM can operate even at a lower supply voltage.
Normally, in writing data on an EEPROM, a high voltage is produced between the drain region and control gate electrode to create hot electrons in part of the channel region near the drain region. Then, those hot electrons are accelerated and injected into the floating gate electrode. To erase data from an EEPROM on the other hand, the charges existing in the floating gate electrode are ejected into the source, drain or channel region by way of the tunnel insulating film.
Hereinafter, a known split-gate semiconductor memory will be described with reference to FIG. 7.
As shown in FIG. 7, the semiconductor memory has memory and logic circuit regions 100 and 200 obtained by partitioning a semiconductor substrate 101 of silicon by an isolation film 102.
On the memory circuit region 100 of the substrate 101, a control gate electrode 104 has been formed with a gate insulating film 103 interposed therebetween. As shown in FIG. 7, the surface of the substrate 101 has a step 101a in the memory circuit region 100. And on the side face of the control gate electrode 104 closer to the step 101a, a floating gate electrode 105 has been formed to cover the step 101a with an insulating film 106 interposed therebetween.
Source/drain regions 107 and 108 have been defined in parts of the substrate 101 beside the control and floating gate electrodes 104 and 105, respectively. Parts of the control and floating gate electrodes 104 and 105 and source/drain regions 107 and 108, which would be exposed otherwise, are covered with a metal silicide film 110.
On the logic circuit region 200 of the substrate 101, a gate electrode 112 has been formed with a gate insulating film 111 interposed therebetween. A sidewall insulating film 113 has been formed on the side faces of the gate electrode 112. Source/drain regions 114 have been defined in parts of the substrate 101 beside the gate electrode 112. And parts of the gate electrode 112 and source/drain regions 114, which would be exposed otherwise, are also covered with the metal silicide film 110.
In the known structure with the memory and logic circuit regions 100 and 200 on one substrate 101, the control and floating gate electrodes 104 and 105, source/drain regions 107, 108 and 114 and gate electrode 112 have their upper surface covered with the metal silicide layer 110 entirely. Accordingly, the devices formed in the memory and logic circuit regions 100 and 200 can operate faster.
In the known memory, however, the control and floating gate electrodes 104 and 105 are adjacent to each other with the extremely thin insulating film 106 interposed therebetween. Thus, when the respective upper surfaces of the control and floating gate electrodes 104 and 105 are silicided, these electrodes 104 and 105 might be short-circuited with each other.
It is therefore an object of the present invention to further reduce the size of, and further enhance the performance of, a semiconductor memory of a split-gate type including memory and logic circuits on the same substrate.
To achieve this object, according to the present invention, the control and floating gate electrodes for the memory circuit do not have their upper surface silicided.
Specifically, a first inventive semiconductor memory includes: a control gate electrode formed over a first active region of a semiconductor substrate with a control gate insulating film interposed therebetween; and a floating gate electrode formed adjacent to a side face of the control gate electrode and over the first active region. A capacitive insulating film is interposed between the side face of the control gate electrode and the floating gate electrode, while a tunnel insulating film is interposed between the first active region and the floating gate electrode. The memory further includes: first source/drain regions defined in parts of the first active region beside the control and floating gate electrodes, respectively; and a gate electrode formed over a second active region of the substrate with a gate insulating film interposed therebetween. The second active region is electrically isolated from the first active region. The memory further includes second source/drain regions defined in respective parts of the second active region beside the gate electrode. In this memory, only the second source/drain regions and the gate electrode have their upper surface covered with a metal silicide film.
In the first inventive memory, only the second source/drain regions and gate electrode of each logical element in the second active region have their upper surface covered with a metal silicide film. That is to say, the control and floating gate electrodes of each storage element in the first active region do not have their upper surface silicided. Accordingly, the control and floating gate electrodes will not be short-circuited with each other. In addition, the upper surface of the first active region, or the memory circuit region, is not silicided. Thus, in forming contacts to electrically connect the first source/drain regions to external members, a decreased mask overlay margin is allowed, so the first active region can be downsized drastically. As a result, a semiconductor memory, including storage and logic elements for memory and logic circuits on the same semiconductor substrate, can have its size reduced considerably and its performance enhanced greatly.
A second inventive semiconductor memory includes: a control gate electrode formed over a first active region of a semiconductor substrate with a control gate insulating film interposed therebetween; and a floating gate electrode formed adjacent to a side face of the control gate electrode and over the first active region. A capacitive insulating film is interposed between the side face of the control gate electrode and the floating gate electrode, while a tunnel insulating film is interposed between the first active region and the floating gate electrode. The memory further includes: first source/drain regions defined in parts of the first active region beside the control and floating gate electrodes, respectively; and a gate electrode formed over a second active region of the substrate with a gate insulating film interposed therebetween. The second active region is electrically isolated from the first active region. The memory further includes second source/drain regions defined in respective parts of the second active region beside the gate electrode. In this memory, only the first and second source/drain regions and the gate electrode have their upper surface covered with a metal silicide film.
The second inventive memory also achieves the effects of the first inventive memory. In addition, the control and floating gate electrodes of each storage element in the first active region do not have their upper surface silicided. Accordingly, the control and floating gate electrodes will not be short-circuited with each other. However, the upper surface of the first source/drain regions in the first active region has been silicided. Thus, the contact resistance of these regions can be reduced.
In one embodiment of the first or second inventive memory, the substrate preferably has a step, which is located under, and covered with, the floating gate electrode. Then, in writing data, hot electrons, existing in a channel region formed under the control gate electrode, can be injected into the floating gate electrode much more efficiently.
A first inventive method for fabricating a semiconductor memory includes the step of a) partitioning the principal surface of a semiconductor substrate into first and second active regions by forming an isolation film in the principal surface. The method further includes the step of b) forming a first insulating film and a first conductor film in this order over the first and second active regions. The method further includes the step of c) selectively etching away parts of the first conductor and first insulating films, which have been located over the first active region, thereby forming a control gate insulating film and a control gate electrode out of the first insulating and first conductor films, respectively, over the first active region. The method further includes the step of d) forming a second insulating film and a second conductor film in this order over the first active region as well as over the control gate electrode. The method further includes the step of e) etching back the second insulating and second conductor films, thereby forming a floating gate electrode out of the second conductor film adjacently to a side face of the control gate electrode with the second insulating film interposed therebetween. The method further includes the step of f) selectively etching away parts of the first conductor and first insulating films, which have been located over the second active region, thereby forming a gate insulating film and a gate electrode out of the first insulating and first conductor films, respectively, over the second active region. The method further includes the step of g) defining first source/drain regions in parts of the first active region beside the control and floating gate electrodes, respectively. The method further includes the step of h) defining second source/drain regions in respective parts of the second active region beside the gate electrode. And the method further includes the step of i) depositing a passivation film over the first active region and then siliciding the respective upper surfaces of the second source/drain regions and the gate electrode using the passivation film as a mask.
According to the first inventive method, the control and floating gate electrodes and first source/drain regions of each storage element in the first active region do not have their upper surface silicided. Accordingly, the control and floating gate electrodes will not be short-circuited with each other. In addition, the upper surface of the first active region, or the memory circuit region, is not silicided, either. Thus, in forming contacts to electrically connect the first source/drain regions to external members, a decreased mask overlay margin is allowed, so the first active region can be downsized drastically. As a result, a semiconductor memory, having the first and second active regions as memory and logic circuit regions on the same semiconductor substrate, can have its size reduced considerably and its performance enhanced greatly.
A second inventive method for fabricating a semiconductor memory includes the step of a) partitioning the principal surface of a semiconductor substrate into first and second active regions by forming an isolation film in the principal surface. The method further includes the step of b) forming a first insulating film and a first conductor film in this order over the first and second active regions. The method further includes the step of c) selectively etching away parts of the first conductor and first insulating films, which have been located over the first active region, thereby forming a control gate insulating film and a control gate electrode out of the first insulating and first conductor films, respectively, over the first active region. The method further includes the step of d) forming a second insulating film and a second conductor film in this order over the first active region as well as over the control gate electrode. The method further includes the step of e) etching back the second insulating and second conductor films, thereby forming a floating gate electrode out of the second conductor film adjacently to a side face of the control gate electrode with the second insulating film interposed therebetween. The method further includes the step of f) selectively etching away parts of the first conductor and first insulating films, which have been located over the second active region, thereby forming a gate insulating film and a gate electrode out of the first insulating and first conductor films, respectively, over the second active region. The method further includes the step of g) defining first source/drain regions in parts of the first active region beside the control and floating gate electrodes, respectively. The method further includes the step of h) defining second source/drain regions in respective parts of the second active region beside the gate electrode. And the method further includes the step of i) depositing a passivation film over the control and floating gate electrodes and then siliciding the respective upper surfaces of the first and second source/drain regions and the gate electrode using the passivation film as a mask.
According to the second inventive method, the control and floating gate electrodes of each storage element in the first active region do not have their upper surface silicided. Accordingly, the control and floating gate electrodes will not be short-circuited with each other. However, the upper surface of the first source/drain regions in the first active region is silicided. Thus, the contact resistance of these regions can be reduced.
In one embodiment, the first or second inventive method preferably further includes the step of forming a step at part of the surface of the substrate, where the floating gate electrode will be formed, between the steps c) and d) so that the step extends along the width of the control gate electrode.
In another embodiment of the first or second inventive method, the step h) of defining the second source/drain regions is preferably performed after the step g) of defining the first source/drain regions has been performed. Then, the second source/drain regions can be defined to have a junction shallower than that of the first source/drain regions. Accordingly, the elements in the second active region (i.e., the logic circuit region) can have their performance further enhanced. As a result, a split-gate semiconductor memory of even higher performance is realized.
In still another embodiment of the first or second method, the step i) of forming the passivation film may include covering the gate electrode as well with the passivation film. And the method may further include the step of forming a sidewall insulating film on the gate electrode out of parts of the passivation film that are located on the side faces of the gate electrode. Then, the process step of forming a sidewall insulating film over each element in the second active region can be omitted and the fabrication process can be simplified.